This invention relates to a corrosion resistant lid or cover for closing a semiconductor package and to a method or process of forming such a cover or lid. The invention is particularly directed to an improvement over multilayer electroplated metal lids.
In many semiconductor applications, the semiconductor device is required to be hermetically sealed within a "package" or housing which protects the device from the surrounding ambient and thus enhances its reliability. As explained in further detail in U.S. Pat. Nos. 3,340,602; 3,538,597; 3,874,549 and 3,946,190, the package typically includes a container having a cavity in which the device is securely seated. Electrical leads are passed out of the container and are connected to appropriate circuitry. The package is closed using a lid which is placed in registration over the cavity and sealed in place using a eutectic solder frame formed of an alloy of 80% gold and 20% tin.
The lid that is used throughout the industry is typically made from a Kovar stamping. Kovar is a well known trade name that identifies an alloy containing various amounts of cobalt, nickel and iron. Lids have typically been provided with a top coating of gold over an inner layer of nickel. The coating provides an excellent bonding surface for the solder frame and also provides a corrosion resistant shield for the Kovar substrate which, because it contains iron, is highly susceptible to rust damage. The nickel interface usually consists of between 50 and 350 microinches of low stress nickel while the top coat consists of about 50 microinches of pure gold. Although this dual combination exhibits good solderability, the lid nevertheless will rust when exposed to a corrosive atmosphere for any period of time. Corrosion in amounts of between 2-4% of the total surface area of the lid will generally occur within 24 to 96 hours when the coated lid is exposed to a salt-containing atmosphere.
The accepted standards in the industry which governs the amount of corrosion allowable for high reliability packages is set out in the military specification Mil. Std. 883 C. This specification has been recently revised so that all lids now must remain corrosion free (zero corrosion) after being exposed to a salt containing atmosphere for at least 24 hours. Dual coated lids found in the prior use, i.e., those having only a single coating of nickel and a top coating of gold, continually fail the corrosion test as set out in these specifications.
All electroplated metal coatings exhibit porosity to some extent and thus permit rust producing atmospheres to pass therethrough to the base metal. Methods have been tried with varying degrees of success to reduce the porosity of protective coatings and to increase the resistance of these lids to corrosion. Porosity is usually inversely proportional to the thickness of an electroplated metal and the pores that initially form in the coating material close gradually as more metal is deposited. As the coating thickens, the pores eventually close. Approximately 2000 microinches of nickel and about 100 microinches of good are required, however, to completely close the pores on a dual coated lid. (See Harper, Charles A., Handbook of Materials and Processing for Electronics. McGraw Hill, 1970, p. 10-56). The consumption of this amount of metal is not only expensive, but also requires an extraordinary amount of production time to complete the plating process. Typically, the nickel electroplate requires four to six hours for each 100 microinches of nickel.
Pulse plating has also been tried with some limited success in an effort to close the pores in the coating materials. In this process, the current applied to the electroplating tank is pulsed on and off periodically by a square wave generator. The pulsing provides for increased ion mobility in the bath which, in turn, results in a smaller more densely packed crystal structure in the electroplated metals. This denser deposit is believed to fill the pores more rapidly and thus provide greater protection for a given coating thickness. Although the amount of corrosion may be reduced by this technique, pulse coating alone cannot provide economically feasible products capable of meeting the new standard within the industry.
A multilayer electroplated Kovar plate cover or lid has been proposed, e.g. in U.S. Pat. No. 4,601,958 and in U.S. Pat. No. 4,620,661. In such a lid, the Kovar substrate has a multilayered protective coating electroplated on it which comprises a first layer of nickel, a second layer of a noble metal or an alloy containing a noble metal, a third layer of nickel and a top layer of gold. The multilayered coating serves to not only close the pores in the coating structure and thus enhance the lid's resistance to corrosion, but also preserves the lid's ability to be hermetically sealed to the package. These lids or covers have been quite successful in meeting or exceeding the specifications of military specifications Mil. Std. 883 C. The multilayer coating achieves a superiority in porosity, and thus is less penetrable by corrosive electrolytes so that the Kovar base metal will not oxidize and corrode. Nevertheless, this multilayer system does have a few drawbacks.
Because at least four distinct electroplating operations must be carried out on the Kovar substrate, at least twice as much time is required to complete the production as is required for a two layer electroplating operation.
The intermediate gold layer, being sandwiched between nickel layers, is impossible to recover by commercially economic means. In the event that the lids or covers are rejected and must be scrapped, only the outer gold layer can be salvaged from the lids. No economic process exists to strip the second nickel layer, so the first layer of gold or other noble metal cannot be recovered from the rejected lids.
The second nickel layer can be blistered, and lose adhesion from the gold layer beneath, due to stressing that is often present under electroplating conditions. The resulting blisters can crack and cause failure of the protective plating and corrosion of the Kovar substrate beneath.
Finally, the porosity of electroplated nickel and gold is rather high compared to worked or cast metals, even in a multilayer system. An ideal corrosion resistant lid should have the crystalline grains of the nickel closely fitting against one another to avoid the pores or gaps at the grain boundaries, but this cannot be reasonably accomplished by electroplating.